A student investigates the rate of reaction between marble chips (calcium carbonate) and hydrochloric acid. She carries out two experiments using the same mass of marble chips and the same volume and concentration of acid, but in one experiment she uses large marble chips and in the other she uses small marble chips. Explain why the experiment using small marble chips produces a faster rate of reaction.

Pearson Edexcel International GCSE Chemistry (4CH1) — 3.2 Rates of reaction · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Small marble chips have a greater total surface area than large chips.
This gives more exposed reacting particles at the surface.
Consequently, collisions between the acid molecules and the calcium carbonate surface occur more frequently.
The higher frequency of collisions leads to a higher rate of successful collisions, so the reaction proceeds faster.

Examiner tips

  • Use the term "surface area" and link it to collision frequency. Show the chain: greater surface area → more reacting sites → more collisions → faster rate. Include "successful collisions" to demonstrate understanding of kinetic theory.

Common mistakes

  • Failing to mention surface area or collision frequency. Using vague phrases like "more contact" without explaining the kinetic effect. Confusing rate of reaction with rate of diffusion or mixing.

Mark scheme (4 marks)

  1. Small marble chips have a greater (total) surface area (than large chips)
  2. There are more exposed reacting particles (at the surface)
  3. Collisions between reactant particles occur more frequently / there are more frequent collisions
  4. There is a higher rate of successful collisions, so the rate of reaction increases

Key terms in this question

rate of reaction

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